C. difficile is a gram positive, anaerobic bacterium that causes gastrointestinal disease in humans. The bacteria are transmitted through feces and spread to food and other surfaces when people who are infected do not thoroughly wash their hands. C. difficile form spores that can persist outside of a human body for weeks or even months. Symptoms of C. difficile infection can range from diarrhea to life-threatening inflammation of the colon. C. difficile infections are the most common cause of infectious diarrhea in the healthcare setting (Cohen S H et al., Infect Control Hosp Epidemiol 2010; 31:431-55).
C. difficile infections are more frequent in older adults in a hospital or long-term care facility and commonly occur during or following antibiotic treatment, which disrupts the normal flora of the gut and permits the opportunistic C. difficile to colonize the gut. In more severe infections, the colon can become inflamed (colitis) or form patches of raw tissue that can bleed or produce pus (pseudomembranous colitis). Symptoms of severe C. difficile infection include watery diarrhea, abdominal cramping and pain, nausea, fever, dehydration, and weight loss.
C. difficile produces two cytotoxic enterotoxins, toxin A and toxin B, that have been identified as targets for therapeutic intervention. Toxins A and B are released by the bacteria into the gut and believed to be involved in causing C. difficile-associated disease (CDAD) or the symptoms associated with CDAD. Symptoms of CDAD can be reproduced in animal models by transfer of the toxins. Toxins A and B have glucosyl transferase activity, which is capable of transferring glucose residues from UDP-glucose to Rho-GTPases, thereby inactivating the GTPase proteins found inside the target host cell. Inhibition of the Rho-GTPases results in depolymerization of actin filaments within the host cell, leading to dysregulation of actin cytoskeleton and tight junction integrity, which in turn produces increased cell permeability and loss of barrier function, diarrhea, inflammation, and an influx of molecules associated with the innate immune response. Toxins A and B are found in fecal samples and can be used to diagnose C. difficile infection.
Once a C. difficile infection has been identified, it is best, if possible, to stop taking the antibiotic that caused the infection. The typical treatment for C. difficile is another antibiotic, usually metronidazole or fidaxomicin, for mild to moderate illness, or vancomycin for more severe symptoms. If effective, these antibiotics prevent C. difficile from growing and allow the normal flora to return and colonize the gut. However, in recent years, strains resistant to these antibiotics have been identified, as well as higher recurrence or reinfection rates. Another approach is taking probiotics. Probiotics are non-pathogenic microorganisms, such as bacteria or yeast that compete with C. difficile and help restore balance to the intestinal tract. For patients with severe pain or inflammation, another option is surgery to remove the diseased portion of the colon.
Therapeutic antibodies have been a rapidly emerging field in recent years and provide another possible strategy for treating C. difficile infections. Patients infected with C. difficile experience a wide range of symptoms, the reasons for which are not fully understood. However, antibodies may play a role, as patients who experience milder symptoms tend to possess high titers of anti-toxin A antibody serum titers, while patients susceptible to recurring infections have demonstrated low titers of circulating anti-toxin A antibodies (Hussack and Tanha, Toxins, 2010, (2):998-1018). US2012/269841 describes murine antibodies that bind mutant C. difficile toxin-A or anti-toxin B. WO2011/130650 describes murine anti toxin-A and anti-toxin B antibodies that were optionally humanized to reduce their immunogenicity, including the lead anti-toxin A antibody, PA-50, and the lead anti-toxin B antibody, PA-41. U.S. Pat. No. 8,257,709 describes anti toxin-A and anti-toxin B antibodies that were generated in transgenic mice, including the lead anti-toxin A antibody, 3D8, and the lead anti-toxin B antibody, 124-152. The transgenic mice contain human immunoglobulin genes encoding certain unrearranged human heavy chain and kappa light chain sequences and, thus, are less immunogenic than murine antibodies.
There remains an unmet need for effective treatment of C. difficile infection, particularly non-invasive treatments that are effective against antibiotic-resistant strains of C. difficile and/or against high-toxin producing strains, including therapeutic antibodies that present reduced immunogenicity while providing high binding affinity for C. difficile toxin A or toxin B and/or potent neutralization activity.